1. Field of the Invention
The present invention relates to a surface acoustic wave element having bump electrodes. More specifically, the present invention relates to a surface acoustic wave element having bump electrodes suitable for packaging while applying ultrasonic waves at joint portions thereof.
2. Description of the Related Art
In following recent trends toward miniaturization and thinning of electronic components, packaging of the electronic components via face-down bonding, where the functional surface of a surface acoustic wave element is packaged by arranging the functional surface so as to directly oppose to the packaging surface of a substrate, has been developed.
The general construction of the surface acoustic wave device packaged via face-down bonding is described with reference to FIG. 1 to FIG. 3. FIG. 1 shows a plan view of a surface acoustic wave element. FIG. 2 shows a cross section of the surface acoustic wave device in which the surface acoustic wave element is packaged. FIG. 3 shows an enlarged cross section of a package of the surface acoustic wave element in the area of a bump electrode. As shown in FIG. 1, the surface acoustic wave element 12 includes a piezoelectric substrate 21, a comb electrode 20 provided on the piezoelectric substrate 21, where the comb electrode 20 is made of a conductive thin film primarily including Al, a reflector electrode 19, an input electrode 16, an output electrode 17, and ground electrodes 18. Among these electrodes, the input electrode 16, the output electrode 17, and the ground electrodes 18 also function as electrode pads for supplying a high frequency voltage. Referring to FIG. 2, the bump electrodes 11 are provided on the electrode pads 16, 17 (not shown) and 18. The surface acoustic wave element is connected to a package electrode 13, which is provided on the package 14, via the bump electrodes 11.
The electrodes 16 to 20 provided on the piezoelectric substrate 21 are formed by patterning into a prescribed shape, via photolithography and etching, a metal thin film primarily made of Al having a thickness of about 0.1 to 0.2 xcexcm, which is disposed on the substrate 21 by vacuum deposition or sputtering. Since these electrodes are simultaneously formed by vacuum deposition, the film thickness of the electrodes 16 to 18, or of the electrode pads, is determined by the film thickness of the comb electrode 20. In other words, when the comb electrode 20 is required to have a thickness of about 0.1 to 0.2 xcexcm, the electrode pads 16 to 18 can not be formed to be thicker than the comb electrode. Accordingly, the electrode pads 16 to 18 are very weak. As such, when the bump electrodes 11 are directly formed on the electrode pads 16 to 18 having a thickness of 0.1 to 0.2 xcexcm, and the surface acoustic wave element 12 is packaged on the package 14 via the bump electrodes 11, sufficient bonding strength cannot be obtained causing, for example, peel-off of the electrode pads 16 to 18.
As shown in FIG. 3, an intermediate electrode 22, which is also primarily made of Al, has a thickness of about 1 xcexcm and is provided via vacuum deposition or sputtering on the electrode pads 16 to 18. Thus, sufficient bonding strength is achieved by providing an additional electrode layer on the electrode pads 16 to 18. However, the surfaces of the electrode pads 16 to 18 are easily oxidized since they are made mainly of Al. Consequently, bonding strength between the electrode pads 16 to 18 and the intermediate electrode 22 is insufficient if the intermediate electrode 22 is also made of Al, and if directly provided on the electrode pads 16 to 18 having oxidized surfaces. Accordingly, a base electrode 23 made of Ti, which has good bonding strength with Al is provided at the bottom surface of the intermediate electrode 22 to ensure sufficient bonding strength between the electrode pads 16 to 18 and the intermediate electrodes 22.
Accordingly, a thin layer made of Ti as the base electrode 23 is provided on the electrode pads 16 to 18, an Al layer serves as the intermediate electrode 22, the bump electrode 11 is provided on the intermediate electrode 22, and the surface acoustic wave element is disposed at a position where the package electrode 13 is opposed to the bump electrode 11. These components are packaged on the package 14 by press-bonding while applying ultrasonic waves or heat.
However, the conventional surface acoustic wave element as described above has the following problems. When the surface acoustic wave element 21 is arranged to be opposed to the package electrode 13 provided on the package 14 via the bump electrode 11, and the package electrode 13 is press-bonded to the package electrode 13 while applying ultrasonic waves or heat, the joining portion of the bump electrode 11 of the surface acoustic wave element, or the electrode pads 16 to 18 and the intermediate electrode 22 located thereon, suffers from large stresses. Since bonding strength between the electrode pads 16 to 18 and the intermediate electrode 22 is enhanced by providing the base electrode 23 such as Ti having a high bonding strength with Al at the bottom of the intermediate electrode 22, the stress generated in bonding the package electrode 13 to the bump electrode 11 becomes concentrated on the piezoelectric substrate 21 at the bottom of the electrode pads 16 to 18, not at the joint portion between the electrode pads 16 to 18 and the intermediate electrode 22. Consequently, cracks and breakage are caused on the piezoelectric substrate 21, which cause breakage of elements, decreased bond strength between the elements and the package 14, and breakdown of electrical continuity.
To overcome the problems described above, preferred embodiments of the present invention provide an apparatus and method of manufacturing a surface acoustic wave element that does not have the problems of breakage of elements, decreased bond strength between the elements and the package, or breakdown of electrical continuity when the surface acoustic wave element is arranged opposite to the package electrode while applying ultrasonic waves or heat.
In a preferred embodiment of the present invention, an electronic element includes a substrate with an electrode pad thereon, a base electrode provided on the electrode pad, an intermediate electrode provided on the base electrode, and a bump electrode provided on the intermediate electrode, wherein the base electrode includes a metallic material that reduces orientation of the intermediate electrode.
In another preferred embodiment of the present invention, a surface acoustic wave element includes a piezoelectric substrate, a comb electrode on the piezoelectric substrate, electrode pads on the piezoelectric substrate, base electrodes provided on the electrode pads, intermediate electrodes provided on the base electrodes, bump electrodes provided on the intermediate electrodes, wherein the bump electrodes are made of a metal having a melting point of about 450xc2x0 C. or more. Further, the base electrodes include a metallic material that reduces orientation of the intermediate electrodes.
In another preferred embodiment of the present invention, a method of packaging an electronic element includes the steps of providing a piezoelectric substrate, forming electrode pads on the piezoelectric substrate, disposing base electrodes on the electrode pads, disposing intermediate electrodes on the base electrodes, forming bump electrodes on the intermediate electrodes, wherein the bump electrodes are made of a metal having a melting point of about 450xc2x0 C. or more, and disposing the electronic element on a package such that a bump electrode opposes a package electrode and press-bonding the package electrode to the bump electrode while applying ultrasonic waves or heat. Further, the base electrodes include a metallic material that reduces orientation of the intermediate electrodes.
According to preferred embodiments of the present invention, the base electrode such as NiCr is provided at the bottom of the intermediate electrode in order to reduce orientation of Al or an alloy containing Al that constitutes the intermediate electrode. When the surface acoustic wave element is arranged opposite to a package electrode on the package via the bump electrode, and when the element is press-bonded while applying ultrasonic waves or heat, the stress applied to the electrode pad and the substrate is dispersed since Al or an alloy containing Al in the intermediate electrode has a low degree of orientation. Thus, the substrate does not generate cracks and breakage.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.